This invention relates generally to electric drive systems and, more particularly, to a battery load leveling system that may be utilized with a hybrid or an electric vehicle.
At least one known vehicle includes batteries, typically lead-acid batteries, to provide electric power for vehicle propulsion. For example, FIG. 1 is a prior art vehicle that includes a conventional alternating current (AC) electric propulsion system 10 that is used in electric propelled and also hybrid-electric vehicles. The electric propulsion system includes at least one energy storage unit such as a lead acid battery 12, and a direct current (DC) contactor 14 to electrically disconnect the energy storage unit 12 from a traction inverter 16 via a DC link 18. Specifically, the battery 12 is connected to a DC link which connects to a frequency controlled inverter such as traction DC-AC inverter 16 for controlling power to an AC motor 20.
In the operation of the vehicle, the battery is often called upon to deliver short bursts of power at high current levels, typically during acceleration of the vehicle or while operating the vehicle up a steep grade, for example. When high current is drawn from conventional batteries, battery terminal voltage drops. Such voltage reduction can interfere with proper operation of the vehicle or reduce efficiency of the switching devices in the power control circuit since the control circuit must also be designed to operate at high efficiency at full battery voltage, i.e., when the vehicle is drawing nominal current in a constant speed mode.
One method for reducing the effect of high current requirements on electric drive system batteries is to use an auxiliary passive energy storage device coupled to the DC link to provide additional power during high current situations. One implementation of this method is shown in the prior art FIG. 2. Specifically, FIG. 2 illustrates an energy storage system 30 that includes a traction battery 32 and an ultracapacitor 34, and a relatively low-cost ultracapacitor electronic interface 36 that allows the ultracapacitor 34 to share power with the traction battery 32 during vehicle acceleration and other high power demands while climbing steep grades.
During operation, when the known vehicle is operated during a lower power cruise condition, a diode 40 allows the ultracapacitor voltage to remain at a slightly higher voltage than the battery voltage Immediately after the high power acceleration is complete, the required current from the energy storage system substantially decreases and the battery voltage increases to the nominal battery voltage or possibly higher voltages, while the ultracapacitor remains at approximately the voltage immediately after the acceleration. Moreover, when the vehicle is decelerating, a silicon-controlled rectifier 42 is gated and the regenerative energy from the electric motor 44 and associated traction drive 46 initially charges the ultracapacitor 34 until the voltage increases to a point where the diode 40 conducts, at which point both the ultracapacitor 34 and battery 32 are partially recharged. As such, the known energy storage system functions quite well and also provides an efficient and low-cost interface between the ultracapacitor and the traction battery in low speed electric vehicles, including golf cars and small utility vehicles. However, during operation, the amount of energy stored in the ultracapacitor 34 is limited by the square of the voltage difference between the maximum battery voltage plus the voltage drop across the diode 40, and the minimum battery voltage plus the voltage drop across the diode 40.
As a result, known energy storage systems are less effective for providing relatively high power levels over an extended period of time while the vehicle is either accelerating under heavy loads and/or climbing steep grades. Moreover, known energy systems include an electronic interface that may be less effective in matching the ultracapacitor output voltage with the voltage level required for the traction drive with acceptable efficiency and at a reasonable cost.